The Influence of Protein-Lipid Interactions on Signal Transduction Litman, B. G protein-coupled receptors are ubiquitous components of signal transduction systems. This project includes an assessment of the role of polyunsaturated phospholipids in modulating G protein-coupled signal transduction and an elucidation of the mechanism of action of ethanol and general anesthetics in these systems. The visual transduction pathway, a prototypical G protein-coupled system, is being used as a model system. The effect of alcohols, anesthetics, and lipid composition on: the kinetics and extent of formation of metarhodopsin II (MII), the G protein activating form of rhodopsin; MII/G protein complex formation; the rate of G protein activation; cGMPphosphodiesterase activation; and the GTPase activity of the G protein are being studied. Short chain alcohols, such as ethanol, are found to be stimulatory, in that they promote MII formation, while longer chain alcohols, such as decanol, are inhibitory. Intermediate length alcohols show a smooth transition from excitatory to inhibitory. The alcohols exert their effect in a lipid dependent fashion, showing a greater range and magnitude of effects in a 16:0,22:6n-3 PC than in 16:0,18:1n-9 PC. The effects in the disk membrane were similar to those observed in 16:0,22:6n-3 PC, consistent with the disk membrane lipid containing about 50 percent of its acyl chains as 22:6n-3. The effects of ethanol, acyl chain composition, and cholesterol are well correlated with changes in phospholipid acyl chain packing free volume, as characterized by the time-resolved fluorescence anisotropy behavior of the membrane probe, diphenylhexatriene. Our observations are best explained by a novel lipid packing model developed in this laboratory, in which the presence of polyunsaturated acyl chains, in mixed saturated-unsaturated acyl chain phospholipids, leads to the formation of lateral domains or clusters in the surface of the membrane. The enhanced meta II formation observed in response to acute ethanol exposure would result in hyperactivity of this pathway, whereas the loss of unsaturated acyl chains in membrane phospholipids observed in chronic exposure to ethanol would be expected to diminish the effect of ethanol on the signalling pathway and may in this way contribute to the tolerance to ethanol developed in chronic ethanol exposure.